The Black Mountains turtlebacks: Rosetta stones of Death ... · The Black Mountains turtlebacks: Rosetta stones of Death Valley tectonics Marli B. Miller a,*, Terry L. Pavlis b a

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Earth-Science Reviews 7

The Black Mountains turtlebacks:

Rosetta stones of Death Valley tectonics

Marli B. Miller a,*, Terry L. Pavlis b

a Department of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, United Statesb Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, United States

Accepted 1 April 2005

Abstract

The Black Mountains turtlebacks expose mid-crustal rock along the western front of the Black Mountains. As such, they

provide keys to understanding the Tertiary structural evolution of Death Valley, and because of the outstanding rock

exposure, they also provide valuable natural laboratories for observing structural processes. There are three turtlebacks: the

Badwater turtleback in the north, the Copper Canyon turtleback, and the Mormon Point turtleback in the south. Although

important differences exist among them, each turtleback displays a doubly plunging antiformal core of metamorphic and

igneous rock and a brittle fault contact to the northwest that is structurally overlain by Miocene–Pleistocene volcanic and/or

sedimentary rock.

The turtleback cores contain mylonitic rocks that record an early period of top-southeastward directed shear followed by top-

northwestward directed shear. The earlier formed mylonites are cut by, and locally appear concurrent with, 55–61 Ma pegmatite.

We interpret these fabrics as related to large-scale, basement-involved thrust faults at the turtlebacks, now preserved as areally-

extensive, metamorphosed, basement over younger-cover contacts.

The younger, and far more pervasive, mylonites record late Tertiary extensional unroofing of the turtleback footwalls from

mid-crustal depths. Available geochronology suggests that they cooled through 300 8C at different times: 13 Ma at Badwater; 6

Ma at Copper Canyon; 8 Ma at Mormon Point. At Mormon Point and Copper Canyon turtlebacks these dates record cooling of

the metamorphic assemblages from beneath the floor of an ~11 Ma Tertiary plutonic complex. Collectively these relationships

suggest that the turtlebacks record initiation of ductile extension before ~14 Ma followed by injection of a large plutonic

complex along the ductile shear zone. Ductile deformation continued during extensional uplift until the rocks cooled below

temperatures for crystal plastic deformation by 6–8 Ma. Subsequent low-angle brittle fault slip led to final exposure of the

igneous and metamorphic complex.

The turtleback shear zones can constrain models for crustal extension from map-view as well as cross-sectional perspectives.

In map view, the presence of basement-involved thrust faults in the turtlebacks suggest the Black Mountains were a basement

0012-8252/$ - s

doi:10.1016/j.ea

* Correspondi

E-mail addre

3 (2005) 115–138

ee front matter D 2005 Elsevier B.V. All rights reserved.

rscirev.2005.04.007

ng author.

sses: [email protected] (M.B. Miller), [email protected] (T.L. Pavlis).

M.B. Miller, T.L. Pavlis / Earth-Science Reviews 73 (2005) 115–138116

high prior to late Tertiary extension. In cross-section, the turtleback geometries and histories are most compatible with models

that call on multiple faults rather than a single detachment to drive post-11 Ma extension.

D 2005 Elsevier B.V. All rights reserved.

Keywords: turtlebacks; Death Valley; extension; ductile shear zones; basin; range

1. Introduction

The three Black Mountains turtlebacks have long

been considered important but enigmatic features of

Death Valley geology. They were named bturtlebacksQby Curry (1938) because their convex-upwards mor-

phologies resemble turtleshells. In terms of bedrock

geology, however, the turtlebacks stand out because

they offer the only exposures of crystalline basement

rock along the Black Mountains front (Curry, 1938,

1954; Fig. 1). They are probably the single most

important features in deciphering Death Valley’s com-

plex tectonic story.

Each of the turtlebacks displays the essential fea-

tures of Cordilleran Metamorphic Core Complexes

(Davis, 1980; Lister and Davis, 1989): a ductilely

deformed metamorphic core, an overlying body of

highly faulted upper crustal rock, and at their north-

west margins, a brittle extensional fault zone between

them. Additionally, mylonitic rocks of the core reflect

a sense-of-shear that is similar to, but older than, the

fault zone. Post- to syn-mylonitization folding about

axes subparallel to shear zone transport produced up to

4 km of structural relief both at high angles and parallel

to transport. Thus, each turtleback individually repre-

sents a bcore complexQ that occupies an area of only

about 6 km2, but collectively, the turtlebacks represent

different parts of a large-scale extensional system. This

complex 3D geometry provides outstanding opportu-

nities to observe both shallow- and moderately deep-

level processes of shear zone evolution.

The Death Valley region is an especially fertile

area for testing models for crustal extension. There,

tectonic interpretations generally fall into one of

two categories (Fig. 2). One category, the bRollingHingeQ model (Stewart, 1983; Hamilton, 1988; Wer-

nicke et al., 1988; Snow and Wernicke, 2000), calls

for ~80 km of horizontal translation and unroofing

of the Black Mountains on a detachment fault of

regional extent. Because the Black Mountains con-

tain ~11 Ma plutons, this faulting must postdate 11

Ma. This model views strike-slip faults, such as the

en echelon Furnace Creek and Sheephead faults, as

upper crustal edges of the detachment system. The

other category calls for extension by slip on numer-

ous, distinct fault zones (Wright and Troxel, 1984;

Wright et al., 1991; Serpa and Pavlis, 1996; Miller

and Prave, 2002). This category, the bPull-ApartQmodel, calls on the strike-slip faults to penetrate

deeply into the crust and drive extension between

their terminations, similar to the model proposed by

Burchfiel and Stewart (1966) for modern Death Val-

ley. Both interpretations rely heavily on findings

from the turtlebacks because they each describe the

turtlebacks as the principal shear zones. In the Roll-

ing Hinge model, the turtlebacks are different expo-

sures of the same detachment fault; in the Pull-Apart

model, the turtlebacks define three of the largest, and

distinct, faults.

Both tectonic interpretations also rely on differing

views of the original configuration of the Mesozoic

fold-thrust belt. The Rolling Hinge model requires an

originally narrow thrust belt in which the Panamint

and Chicago Pass thrusts, now separated by about 80

km, were originally the same structure. Detachment

faulting during late Tertiary extension cut this fault

and displaced the two parts to their present locations

(Figs. 1 and 2A). By contrast, the Pull-Apart model

requires an originally wider thrust belt in which the

Panamint and Chicago Pass thrusts originated as sepa-

rate features (Fig. 2B).

The turtlebacks are here described as bRosettaStonesQ because they provide keys to decipher both

the original fold-thrust belt geometry as well as the

geometry and mechanisms of late Tertiary extension.

Their locations at the western edge of the Black

Mountains place them in the middle of the fold and

thrust belt. Most recently, Miller and Friedman (1999)

and Miller (2003) have found evidence in their foot-

walls for mid-crustal, pre-55 Ma, basement-involved

thrust faulting. The addition of the Black Mountains

to the fold-thrust belt helps constrain its original

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QalQuaternary alluvium

PQal Pliocene-Quaternary deposits

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ZPz Paleozoic and upper Proterozoic sedimentary rocks

pCp Late Precambrian Pahrump Group

Sediments and Sedimentary Rocks

Igneous and metamorphic rocks

Tvv v

v v Volcanic rocks,

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Tertiary intrusive rocks

Mzi Mesozoic intrusive rocks

pC Precambrian gneiss

mostly late Tertiary

undifferentiated

Willow Spring pluton

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Fig. 1. Map of the central Death Valley region; inset shows close-up view of Black Mountains. Abbreviations: AC—Amargosa Chaos; AF—

Amargosa Fault; B—Badwater turtleback; Bbm—Billie Borate Mine; BM—Black Mountains; BS—Badwater Spring; C—Copper Canyon

turtleback; CM—Cottonwood Mountains; CP—Chicago Pass thrust; FM—Funeral Mountains; HMB—Hunter Mountain batholith; L—

Lemoigne thrust; M—Mormon Point turtleback; MC—Marble Canyon thrust; NR—Nopah Range; P—Panamint thrust; PM—Panamint

Mountains; RS—Resting Spring Range; S—Smith Mountain.

M.B. Miller, T.L. Pavlis / Earth-Science Reviews 73 (2005) 115–138 117

geometry to one that favors the Pull-Apart model and

severely limits the Rolling Hinge model.

The turtlebacks therefore offer world-class field

laboratories to study the processes of structural geol-

ogy and tectonics—the former through their superb

exposures of mid- and upper-crustal fault zones and

associated fault rocks and structures, and the latter

through the insights they can give to models of crustal

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A

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Detachment fault

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Fig. 2. Schematic cross-sections from A to AV of Fig. 1 that compare interpretations of present-day structure of Death Valley region.

Abbreviations: CP—Chicago Pass thrust; HMB—Hunter Mountain batholith; L—Legmoigne thrust; MC—Marble Canyon thrust; P—Panamint

thrust; TB—Turtlebacks. BMT and TB are both projected into line of section. A: Present-day structure as interpreted in context of the Rolling

Hinge model by Wernicke et al. (1988) and Snow and Wernicke (2000). Note that extensional turtleback faults (TB) are part of a regional

detachment. Stars mark correlative structures offset by the detachment. Dashed line beneath Nopah range indicates that part of the detachment

system underlies the Nopah Range as well. B: Present-day structure as interpreted in context of Pull-Apart model. Note that extensional

turtleback faults are deep-seated faults.


extension. This paper reviews contributions made by

researchers on the turtlebacks and concludes with our

preferred interpretation of the turtlebacks as being

long-lived shear zones that probably reactivated base-

ment-involved thrust faults.

2. Research on the turtlebacks

Curry (1938) first described the crystalline cores

of the turtlebacks, their fault zones, and their dis-

tinctive antiformal geometries. From north to south,

Curry (1938, 1954) described three turtlebacks in the

Black Mountains, the Badwater, Copper Canyon, and

Mormon Point turtlebacks (Figs. 1 and 3A,B,C). He

Fig. 3. Photographs of the Black Mountains turtlebacks. In each, view is to

turtleback; M—Mormon Point turtleback. A: View of Badwater turtleback

Fig. 7A. Arrow marks location of Fault #2 of the Badwater turtleback fau

wall is red and tan. Arrow marks location of Copper Canyon turtleback fa

greenish rock marks basement gneiss.

and Noble (1941) both noted the geometrical simila-

rities between the turtlebacks and the Amargosa fault

near Virgin Spring, in the southern Black Mountains,

and suggested their common origin as a regional

thrust fault. Drewes (1959) argued that the fault

zones were Pliocene or Pleistocene gravity-driven

features, imposed on the turtlebacks long after thrust

faulting and folding. His map of the central and

southern Black Mountains (Drewes, 1963) gave the

first detailed look of the complicated structure and

hanging wall stratigraphy of the Mormon Point and

Copper Canyon turtlebacks. Wright et al. (1974) first

recognized that the turtleback fault zones were

rooted normal faults that played an important role

in Death Valley’s late Cenozoic extensional story.

north. Abbreviations: B—Badwater turtleback; C—Copper Canyon

. Fold symbol and letter bcQ corresponds to fold axes as depicted on

lt system. B: Copper Canyon turtleback. Footwall is green; Hanging

ult. C: Mormon Point turtleback. Tan color marks footwall marble;



Otton (1976, 1977) mapped the Mormon Point and

Copper Canyon turtlebacks at 1:24,000 and de-

scribed most of the relations that are key to inter-

pretations today.

A common theme of this early research is the

importance of pre-Cenozoic deformation in shaping

the turtlebacks. Wright et al. (1974) and Otton (1976),

for example, recognized the relevance of the turtle-

backs to crustal extension, but called on Mesozoic

shortening to form their antiformal geometries. How-

ever, with the recognition of the extensional nature of

metamorphic core complexes and their associated

ductilely deformed rocks elsewhere (e.g. Davis et

al., 1982, 1986; Miller et al., 1983) as well as the

recognition that the Willow Spring pluton in the Black

Mountains was only 11 Ma (Asmerom et al., 1990),

most later workers emphasized the extensional fea-

tures in the turtlebacks (e.g. Miller in Wright et al.,

1991; Pavlis in Wright et al., 1991; Miller, 1992a,b;

Holm et al., 1992). By the mid-1990s, most workers

accepted that Tertiary extension, not Mesozoic short-

ening, was the primary cause of the turtleback geo-

metries (Mancktelow and Pavlis, 1994; Holm et al.,

1994a,b).

Otton (1976), however, mapped older-over-younger

contacts at the Mormon Point and Copper Canyon

turtlebacks, which Holm (1992) interpreted as minor

thrust faults related to the Mesozoic contraction. Using

U–Pb dating of zircons, Miller and Friedman (1999)

found pegmatite at the Badwater turtleback that locally

cross-cut mylonitic fabrics to be 55 Ma; they inter-

preted the older fabrics as related to the Mesozoic

Sevier Orogeny. Miller (2003) later reported more

structural data to propose that a significant basement

thrust existed at the Badwater turtleback in addition to

the thrust faults at the Mormon Point and Copper

Canyon turtlebacks, and concluded that the thrusts

were probably early Tertiary in age. This paper further

develops that argument to show the relevance of these

structures to the pre-extension geometry of the fold-

thrust belt.

Several recent studies of the turtleback fault

zones and their hanging walls have clarified impor-

tant aspects of the northern Black Mountains strati-

graphy, turtleback fault geometries and timing of

slip. Miller (1991) and Keener et al. (1993) found

evidence for successive generations of turtleback

fault zones, and interpreted that they were cut by

present-day faults in front of the range. This inter-

pretation is questioned by Hayman et al. (2003).

Pavlis et al. (1993) and Burchfiel et al. (1995)

interpreted the kinematic framework of the embayed

mountain front at Mormon Point. Greene (1997)

mapped stratigraphy and structures of the hanging

wall of the Badwater turtleback north of Natural

Bridge canyon. Knott (1998) documented Pleisto-

cene slip on the Badwater and Mormon Point turtle-

backs and described fault segmentation along the

length of the Black Mountains front. Nemser (2001)

and Hayman et al. (2003) modeled faulting in the

upper plates of the Badwater and Mormon Point

turtlebacks. Most recently, Dee et al. (2004) found

evidence for two stages of ductile deformation at

the Mormon Point turtleback, the most significant of

which predated 9.5 Ma.

3. Features of the turtlebacks

The lower plate, brittle fault zone, and upper plate

define the three principal components of each turtle-

back (Figs. 3 and 4). As suggested by studies of

metamorphic core complexes elsewhere (e.g. Lister

and Davis, 1989) and documented by Miller

(1992a,b), Holm (1992), Pavlis et al. (1993), Keener

et al. (1993), Nemser (2001), and Hayman et al.

(2003), brittle deformation of the hanging walls fit

into the same extensional kinematic picture as that of

the fault zones, which in turn have approximately the

same sense of shear as the footwall mylonites. Each

component also exhibits to varying degrees the anti-

formal geometries characteristic of the turtlebacks.

The footwalls, however, include Proterozoic and

both Neogene and Paleogene intrusive rock and so

have geologic histories that predate Neogene exten-

sion. Here, each component of the turtlebacks, as well

as their antiformal shapes, will be reviewed separately.

3.1. Turtleback footwalls

Each turtleback footwall consists of quartz-feldspar

gneiss and marble with minor amounts of pelitic

schist. They are intruded by a host of igneous rocks

that date from the early to late Tertiary. Most footwall

rocks, except for the youngest intrusions, exhibit a

range of tectonite fabrics. Consequently, the igneous

turtlebackfault

Fig. 4. Photograph of three principal elements of the turtlebacks as seen at Badwater turtleback. Note listric normal fault in hanging wall directly

above geologist.


rocks have been critical in sorting out episodes of

deformation.

3.1.1. Footwall stratigraphy

The gneiss, with a U–Pb zircon age of 1.7 Ga

(Wasserburg et al., 1959; DeWitt et al., 1984), belongs

to the regional crystalline basement complex of the

Mojave Province (Condie, 1992). The marble and

schist are metasedimentary assemblages that are argu-

ably derived from the overlying miogeoclinal section.

Most workers have traditionally assigned these meta-

sedimentary rocks to the Noonday Dolomite and John-

nie Formations (Wright et al., 1974, 1991; Otton,

1976; Holm, 1992). Most still assign part of the sec-

tion to the Noonday Dolomite, but some now question

the interpretation of the associated, non-dolomitic

rock. In the lowest exposed levels of the Badwater

turtleback, the exposed section of pelitic rock contains

about 50 m of feldspathic quartz mylonite, suggestive

of an arkosic protolith (Fig. 5A). This observation,

plus the relative abundance of calcite marble with

silty laminae, led Miller (1992a,b) to suggest that the

clastic rocks were Crystal Spring Formation, possibly

overlain by Noonday Dolomite. At Mormon Point,

Holm (1992) described diamictite-like gneiss structu-

rally beneath the Noonday(?) marble that he argued

was Kingston Peak Formation. We dispute this inter-

pretation, noting that the bclastsQ are actually deformed

granitic sills.

Intrusive rocks of the footwalls consist of early

Tertiary pegmatite and a variety of mafic to felsic

late Tertiary plutons and dikes. Based on U–Pb zircon

ages, the pegmatite is 55F3 Ma at the Badwater

turtleback (Miller and Friedman, 1999), and 61 Ma

at the Copper Canyon turtleback (R. Friedman, 2000,

personal communication). The pegmatite consists

almost entirely of feldspar and quartz, with minor

muscovite and/or biotite and few accessory minerals.

It is most voluminous at the Badwater turtleback,

where it constitutes approximately 30% of the foot-

wall, but becomes less voluminous southward, where

it constitutes less than 10% of either the Copper

Canyon or Mormon Point turtlebacks. Typically, the

pegmatite forms lens-shaped bodies from 1 m to about

10 m in width that are concordant to foliation in

adjacent rock, but it locally forms dikes. Miller and

Friedman (1999) interpreted the lens-shaped bodies as

boudins and the dikes as parts of locally preserved

braftsQ within ductilely deformed, late Tertiary exten-

sional shear zones. Lee (2003) used geochemistry to

argue that the pegmatites were derived directly from

their metamorphic host rocks.

Feldspathic quartz mylonite

Pelitic schist Pelitic schist

Dolomite marble

Dolomite marble

Fig. 5. Photograph of south wall of deep canyon in Badwater turtleback and stratigraphy of metasedimentary rock.


The late Tertiary intrusions consist of the Willow

Spring pluton, Smith Mountain Granite and associated

felsic bodies, and latite and diabase dikes. These rocks

are described in detail by Holm et al. (1994a,b), Wright

et al. (1991), Otton (1976) and Drewes (1963). The

Willow Spring pluton and the Smith Mountain Granite

are a comagmatic Late Miocene mafic (hornblende

gabbro to quartz diorite) and felsic (biotite rapikivi

granite) assemblage respectively that show magma

mingling textures along a mutually intrusive contact

(Meurer, 1992; Pavlis in Holm et al., 1994a,b). In

addition to this field evidence, geochronological data

from the plutonic complex are broadly supportive of

this conclusion with two U–Pb zircon dates from the

Willow Spring pluton of 11.6F0.2 Ma by Asmerom et

al., 1990 and 10.93F0.75 Ma by DeWitt (reported in

Wright et al., 1991). These ages are only slightly older

than a 10.8 Ma Ar/Ar cooling date on orthoclase from

the structurally highest exposed level of the Smith

Mountain Granite (Pavlis in Holm et al., 1994a,b)

and a U–Pb zircon age of 10.44F0.22 Ma (Miller et

al., 2004).

An important feature of the Late Miocene intrusive

complex is that the intrusives form a sill-like body

intruded along the structural top of each turtleback

(Pavlis in Holm et al., 1994a,b; Pavlis, 1996). This sill

is itself stratified with the mafic Willow Springs

assemblage structurally beneath the felsic Smith

Mountain granite. This geometry is undoubtedly not

coincidental given the evidence that the Smith Moun-

tain and Willow Springs assemblages are phases of

the same plutonic body and so probably reflects den-

sity stratification within the original magma chamber

of the sill. This geometry also led to dramatic thermal

manifestations in the Copper Canyon and Mormon

Point turtlebacks as the hot plutonic sheet above them

cooled (Pavlis, 1996). Effects included: (1) partial to

complete destruction of older fabrics by recrystalliza-

tion; (2) growth of new mineral assemblages sugges-

tive of low-P, upper amphibolite facies conditions

(olivine in marble and muscovite breakdown to form

K-spar+ sillimanite in pelites); and (3) cooling to

biotite closure temperatures at 6–8 Ma (Holm and

Dokka, 1993; Pavlis and Snee, unpublished data),

which is 3–5 m.y. after plutonism, but consistent

with the slower cooling of rocks below a large intru-

sive sheet (e.g. Pavlis, 1996).

3.1.2. Footwall fabrics

Footwall rocks that pre-date the Smith Mountain

granite exhibit tectonite fabrics at all exposed struc-

tural levels in the turtlebacks. These fabrics fall into

two categories: those that involve, and those that are

cut by, the pegmatite. Those that involve the pegma-


tite dominate the footwalls. They show top-to-the-NW

sense-of-shear, consistent with late Tertiary extension

(Fig. 6A). This shear sense determination is unambig-

uous, as it is based on numerous outcrop and micro-

scopic criteria, including shear bands, asymmetric

porphyroclasts and mica fish, and preferred grain-

shape orientations (e.g. Miller, 1992a,b; Holm, 1992).

A late Tertiary age for these fabrics is further

indicated by three other structural and geochronologic

findings. First, the fabrics affect the base of the 11.6

Ma Willow Spring pluton (Holm, 1992; Pavlis in

Wright et al., 1991; Pavlis, 1996). Second, Holm et

al. (1992) obtained Ar–Ar biotite ages of 13 Ma, 6

Ma, and 8 Ma for the Badwater, Copper Canyon and

Mormon Point turtlebacks, respectively, to indicate

when each turtleback cooled through ~300 8C. As300 8C is the approximate temperature required for

crystal-plasticity in quartz (Mitra, 1978; Passchier and

Trouw, 1996), these ages help define when myloniti-

zation ceased in gneissic rocks of each footwall;

calcite-dominated marbles, which require tempera-

tures of about 250 8C (Schmid, 1982), could continue

to deform ductilely. Third, Miller (1992a,b) noted a

continuous evolution of the mylonite zone at the Bad-

water turtleback across the ductile–brittle transition

Fig. 6. Large asymmetric pegmatite clast, deformed with top-northwest (ri

parallel to stretching lineations.

into brittle normal faulting with a clear connection

to Tertiary extension. Miller (1999a,b) then applied

the findings by Holm et al. (1992) to suggest that most

of the deformation at the Badwater turtleback was

between about 16–13 Ma.

At the Mormon Point turtleback, Miller and Fried-

man (2003) obtained a U–Pb zircon age of 9.53F0.04

Ma from silicic dikes that cut most of the shear zone.

Dee et al. (2004) found that these dikes were deflected

into the shear zone and internally deformed at the

highest structural levels. Based on an approximate

458 angle of deflection, they argued for a post 9.5 Ma

angular shear strain of only about one. Pre-9.5Ma shear

strains, however, were likely much greater.

In contrast, early fabrics that are cut by the pegma-

tite appear as locally preserved braftsQ within the late

Tertiary zone. Foliations and lineations of these rocks

are parallel to those of the late Tertiary fabrics and so

can be identified only where pegmatite is present. They

typically show a top-to-the-east sense of shear (Holm,

1992; Turner and Miller, 1999; Miller, 2003) to sug-

gest a pre-late Cenozoic shortening origin. Miller

(2003) suggested they were at least partly coeval

with intrusion of the pegmatite to make them of early

Tertiary age. We argue later in this paper that the

ght) shear sense at the Badwater turtleback. Plane of photograph is


parallelism of both sets of fabrics suggests the turtle-

back shear zones are in fact reactivated thrust faults.

3.1.3. Metamorphism

Each turtleback footwall experienced amphibolite-

to upper amphibolite facies metamorphism prior to

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v

vv

v v

v

A

116°43' W

36°15' N

26

45

54

46

48

48

22

35

32

2419

16

25

35

8A

5A

60

68

71

42

30

20

40

40 28

18

36

16

21

37

26

54

Footwall (undifferentiated)Fault #3

Fault #2

Fault #1

B

B'

A A'

1a

BS

1a

N

B

unroofing by late Tertiary extension. This grade is

evident by the presence of crystal-plastically strained

feldspar, indicative of temperatures greater than

about 450 8C (Tullis and Yund, 1980; Pryer, 1993),

as well as the metamorphic mineral assemblages in

pelitic and carbonate rocks. At the Badwater Turtle-

+

+

+

+

+

+

+

+ ++

+

++

+

+

+

+

+

+ + ++

+

+

1 km

Gneiss

Gneiss

C

M

A

B

WillowSpring pluton

Z

Z

PQal

Ts

116°45' W

36°5' N

de

f

18

35

18

28 26

46

33

29

36

24

30

25

28

25

17

5875

55

512 45

7225

40 7

8B


back, kyanite-bearing schists, in association with

garnet, rutile, ilmenite, and quartz (GRAIL) yielded

a pressure of 700F100 MPa for an approximate

depth of 20 km and temperature between 500 8Cand 675 8C (Whitney et al., 1993). At Mormon

Point, Pavlis (1996) described k-spar and sillima-

nite-bearing schists to indicate much higher tempera-

ture second-sillimanite conditions. These high grade

fabrics at Mormon Point are closely associated with

the Miocene plutonic assemblage, which suggests

that they formed during the Late Miocene (Pavlis,

1996). Furthermore, amphibolite samples collected

from both the Copper Canyon and Mormon Point

turtlebacks contain embayed garnets overgrown by a

rim of plagioclase, a texture that suggests a higher

pressure garnet amphibolite assemblage was over-

printed by a lower pressure, hornblende-plagioclase

assemblage.

To date no samples have been collected at either

Mormon Point or Copper Canyon that contain low-

variance mineral assemblages useful for thermobaro-

metry. However, hornblende geobarometry of the Wil-

low Spring pluton (Holm and Dokka, 1993) and

Smith Mountain Granite (Meurer, 1992) indicates

pressures no greater than 300 MPa in the Copper

Canyon and Mormon Point turtlebacks during the

Late Miocene. These pressures correspond to intru-

sion depths of 10–11 km in the turtleback footwalls

(Holm and Wernicke, 1990; Holm and Dokka, 1993).

Furthermore, Meurer (1992) found that intrusion

depths increased northward in the Willow Spring

pluton, consistent with exhumation along an initially

more steeply dipping fault.

3.1.4. Footwall folds

Each turtleback footwall is the structural culmina-

tion of a northwest–southeast trending, doubly plun-

Fig. 7. Maps and cross-sections of Black Mountains turtlebacks. Dark blu

hinge, whereas red fold symbols indicate locations of apparent fold hinges

shown in circles with arrows that designate view direction. A: Map and c

Spring; N—Natural Bridge Canyon. Badwater Turtleback normal fault

Quaternary alluvium, bvolcanic hanging wall,Q or Willow Spring Pluton. B

separates metasedimentary rock from structurally overlying basement gnei

queried in cross section B-BV. Dip-slip separation on high-angle normal fau

cross-section A-AV shows geometry of brittle fault system projected into li

section. C: Map of Copper Canyon (C) and Mormon Point (M) turtleba

basement gneiss above Noonday Dolomite (Z) exist at location A. Large e

Exposed thrust fault at Mormon Point is marked by heavy line with teeth

ging antiform. These folds are second-generation (F2)

structures relative to the main metamorphic foliation

(S1) in the footwalls, and are composite features

representing different styles of overprinting at differ-

ent positions within large-scale ductile shear zones.

The age of the S1 foliation is unclear, but undoubtedly

includes Mesozoic and/or early Tertiary metamorph-

ism. However, it was clearly re-used and transposed

by S2 during the early stages of the extensional

deformation.

Because the turtleback folds are largely defined by

this foliation, they must have formed during or after

the extension. At Badwater, Miller (1999a,b) applied a

slip-line analysis (Hansen, 1971) to asymmetric minor

folds on the northeast limb to suggest that they formed

within a ductile shear zone during top-to-N 638Wtransport. Each turtleback antiform is locally subpar-

allel to the overlying brittle fault zone, the bturtlebackfaultsQ discussed in the following section. It is this

feature that gives rise to the geomorphic expression

(Fig. 3) first described by Curry (1938) and later

attributed to extensional fault-reactivations of the

foliation by Wright et al. (1974). This generalization,

however, is an oversimplification when each turtle-

back is examined in detail.

Specifically, the principal footwall folds vary con-

siderably from one turtleback to another. At the Bad-

water turtleback, the hingeline is poorly defined, but

the northeastern blimbQ displays abundant outcrop-

scale non-cylindrical folds of the basement gneiss

with metasedimentary rock. These folds typically

trend northwest and range from upright to recumbent.

Mormon Point exhibits a high degree of non-cylind-

rical folding of both the principal antiform and sub-

parallel minor folds. The axial surface of the principal

antiform is nonplanar, as it ranges from nearly upright

at the southeast end of the turtleback to gently inclined

e fold symbols indicate approximate location of footwall antiformal

in fault surface. Locations of photographs in Figs. 5A and 8A,B are

ross sections of Badwater turtleback. Abbreviations: BS—Badwater

shown as line separating gneiss and metasedimentary rock from

adwater Turtleback thrust fault shown by heavy line with teeth that

ss. As bottom of metasedimentary rock is not exposed, its contact is

lt on east side of cross-section B-BV is only approximate. Composite

ne A-AV of map. Fault 1a on map corresponds to fault 1a on cross-

cks, modified from Otton (1976) and Holm (1992). Exposures of

nclave of gneiss that structurally overlies dolomite is at location B.

.


towards the northwest. By contrast, the Copper Can-

yon turtleback only locally displays complex infold-

ing of basement and metasedimentary rock, and

overall exhibits a macroscopic upright to steeply

inclined geometry with a gentle northeast limb and a

steep southwest limb.

To varying degrees, the footwall folds of each

turtleback also display doubly-plunging geometries

(e.g. Miller, 1992b; Mancktelow and Pavlis, 1994;

Pavlis in Holm et al., 1994a,b). As a result, the

relationship between the folds and structurally over-

lying faults varies across each turtleback. To the

southeast, fold hinges plunge southeastward beneath

the structurally overlying Miocene plutonic complex

and/or Precambrian basement (Fig. 7A,B). As re-

ported by Mancktelow and Pavlis (1994), this con-

tact is itself deformed, with ductile fabrics in the

floor of the plutons that are parallel to the those in

the turtleback footwalls. Northwestward, fold hinges

become horizontal and then plunge moderately

northwestward, typically steeper than the structurally

overlying brittle fault. The structurally overlying

brittle faults, in turn, display less structural relief

than the folds. Therefore, at both Copper Canyon

and Mormon Point, the brittle faults cut across struc-

tural section on both limbs of the antiforms to

expose the structurally overlying Miocene plutonic

complex in the core of the synclinoria between them

(Fig. 7B). At the Badwater turtleback, the structu-

rally deepest part of the turtleback is exposed near its

center.

This general form of the turtleback anticlinoria

suggested to Mancktelow and Pavlis (1994) and Pav-

lis (1996) that the folds formed as part of a continuous

deformational sequence related to distributed transten-

sional shear, after intrusion of the plutonic complex.

In this scenario, the transtension created localized

shortening that buckled low- to moderate-angle exten-

sional shear zones (S2), superimposed on the turtle-

back main-phase foliation (S1). This buckling gene-

rated large-scale structural relief in the shear zones,

which ultimately were transected by brittle faults.

Serpa and Pavlis (1996) extended this concept to the

broader regional scale. They noted that the F2 folds in

the turtleback footwalls trend more northerly than

younger folds and so probably reflect greater amounts

of finite clockwise vertical axis rotation. Similarly,

Miller (1999a,b) found that lineations in mylonitic

gneiss trended more northerly than those in calcite

marble at the Badwater turtleback. As the gneiss

reflected earlier, higher temperatures than the marble,

these data may also reflect clockwise rotation during

transtension.

3.1.5. Basement thrust faults

Each turtleback exhibits areally extensive non-pla-

nar contacts where basement gneiss overlies younger,

metasedimentary rock. Miller (2003) interpreted these

contacts as SE-directed, basement-involved thrust

faults that were re-deformed during late Tertiary

extension. At the Badwater turtleback, several hun-

dred meters to approximately 1 km of gneiss overlies

the carbonate at its northern and southern edges

respectively (Figs. 7A and 8A). Near the center of

the Badwater turtleback, thin bodies of gneiss struc-

turally overlie, and are infolded with, carbonate rock.

As the exposed trace of the gneiss-over-carbonate

contact is approximately 4.5 km long in the direction

of transport, Miller (2003) concluded that the thrust

fault had at least that much displacement. At the

Copper Canyon turtleback, small bodies of gneiss

structurally overlie carbonate rock at the contact

with the Willow Spring pluton, and as an enclave

within the pluton (Fig. 7B; Otton, 1976; Holm,

1992). At Mormon Point, ~300 m of gneiss structu-

rally overlies the metasedimentary rock northwest of

Smith Mountain (Figs. 7B and 8B; Otton, 1976;

Holm, 1992).

Miller (2003) further noted exposures of pre- and

syn-pegmatite mylonitic rocks that display top-to-the-

east and -southeast fabrics. These exposures, although

relatively rare, are most abundant at deep structural

levels within each turtleback. As the pegmatite is early

Tertiary in age (Miller and Friedman, 1999) and some

of the mylonitic fabrics appeared to be concurrent

with its intrusion, Miller (2003) argued that both

mylonitization and basement thrusting were probably

early Tertiary.

Except for the presence of cross-cutting pegmatite

and the sense-of-shear, the early mylonitic foliation is

indistinguishable from the late Tertiary, extension-

related foliation. Both sets of foliations are concordant

and both display similarly oriented stretching linea-

tions. In some localities where both foliatons are

preserved together, the later foliation appears to

merge with the older one (Fig. 9). We therefore sug-

A

B

pCg

pCg

Fig. 8. Photographs of exposed older-over-younger contacts at Badwater and Mormon Point turtlebacks. Photograph locations are shown as

b8AQ and b48BQ in Fig. 7A and B, respectively. A: At south side of Badwater turtleback; view toward south. B: On Mormon Point turtleback;

view toward east.


gest that the extensional turtleback shear zones origi-

nated as basement thrust faults, possibly as late as

early Tertiary, but were reactivated and almost com-

pletely transposed during late Tertiary extension.

3.2. Turtleback hanging walls

The delineation of the bhanging wallQ of each

turtleback is somewhat subjective, as the footwalls

NW SE

Fig. 9. Photograph of concordant foliation in pegmatite and previously deformed gneiss. Note that pegmatite clearly cuts across foliation in the

gneiss on the left edge of the photograph and immediately behind the clump of grass. The gneiss also exhibits a much greater intensity of

foliation, but that the foliation continues into the pegmatite. Kinematic indicators from the gneiss yield top-southeastward shear directions; those

from the pegmatite yield top-northwestward shear directions.


of each turtleback plunge northwestward and may

continue for some distance beneath Death Valley.

They also plunge southeastward beneath the Miocene

plutonic complex. The Panamint Range could there-

fore be considered as part of the hanging wall of either

turtleback, whereas the Badwater turtleback could be

considered as part of the Copper Canyon turtleback’s

hanging wall. Alternatively, when viewed to the

south, the turtleback hanging walls consist entirely

of plutonic rock and structurally overlying Proterozoic

basement. This discussion is therefore limited to the

immediate hanging walls: rocks that directly overlie

the visible portions of the brittle turtleback fault

zones.

Numerous studies of the immediate hanging walls

of each turtleback have clarified the ages and types of

rock present, as well as aspects of the structural

geology of each one. In general, each turtleback has

a hanging wall that consists almost entirely of Mio-

cene to Pleistocene sedimentary and/or volcanic rock.

However, significant differences exist between each

one. Similarly, the structures within each hanging wall

reflect aspects of late Tertiary extension, but vary

considerably in specific details.

Rocks of the Badwater turtleback hanging wall

consist of the Miocene to Pliocene Artist Drive

Formation, Greenwater Volcanics (Greene, 1997),

megabreccia deposits of these same rocks, and Pleis-

tocene fanglomerate deposits. The volcanic flows

and interbedded sedimentary rocks lie in the hanging

wall immediately north of the turtleback, whereas

the megabreccia and fanglomerate deposits lie in the

hanging wall immediately west and above the cen-

tral part of the turtleback. To the south, brecciated

rock of the Willow Spring Pluton lies in the hanging

wall.

Miller (1999b) interpreted the megabreccia depos-

its as having originated in landslides, and suggested

that the brittle turtleback fault surface acted as a

gravitational sliding surface in its later history. Fan-

glomerate deposits along the west side of the Bad-

water Turtleback that stratigraphically overlie the

megabreccia deposits provide evidence for Pleistocene

unroofing of the Badwater turtleback (Cichanski,

1990). Deposits of 0.67 Ma Lava Creek B tephra,

as reported by Knott et al. (2001a,b) and Hayman et

al. (2003), establishes their Pleistocene age, while

an increase in the proportion of metamorphic clasts


upwards documents exposure of the footwall to

erosion.

Hanging wall rocks of the Copper Canyon turtle-

back consist, in upwards succession, of Late Miocene

Willow Springs diorite, Tertiary volcanic rocks and

Copper Canyon Formation, and fanglomerate and

megabreccia deposits (Drewes, 1963). The Copper

Canyon Formation dominates the hanging wall. It

consists of approximately 3000 m of alluvial fan

deposits, in fault contact with the turtleback, that

grade laterally (northward) into playa and freshwater

lake deposits, and basalt flows. Holm et al.

(1994a,b), using Ar/Ar, obtained an age of ~6 Ma

to ~3 Ma for its deposition. The Copper Canyon

Formation is significant because it records local

basin development in a transtensional setting, pre-

sumably an early phase of the formation of modern

Death Valley (Ellis and Trexler, 1991). Its fine-

grained deposits also display abundant well-pre-

served Neogene mammal and bird tracks (Curry,

1938; Scrivner and Bottjer, 1986).

At the Mormon Point turtleback, hanging wall

rocks consist of two distinct assemblages (Otton,

1977; Knott et al., 2001a,b): a Pleistocene section

and older Pliocene (?) section. The older Pliocene

(?) rocks dip much more steeply than the Pleistocene

rocks and are limited to a prism of rock bounded by

the turtleback fault and the Willow Wash fault, a

major hanging-wall fault above the turtleback fault.

Pleistocene rocks occur locally as nearly flat lying

deposits atop an angular unconformity on the older

rocks but are primarily exposed as variably tilted,

faulted and exhumed rocks in the hanging wall of

the Willow Wash fault. Burchfiel et al. (1995) made

a detailed structural/stratigraphic map of the area in

which they defined 11 separate units, marked by rapid

facies changes and much interfingering. They argued

that the sediments were deposited along the margin of

modern Death Valley and later uplifted as the range-

bounding normal fault stepped out into the playa.

Knott et al. (2001a,b) and Hayman et al. (2003)

reported the presence of the 0.76 Ma Bishop Ash

and 0.67 Ma Lava Creek B Ash within the younger

deposits providing the first clear evidence of a Pleis-

tocene depositional age.

Structures in the turtleback hanging walls consist

predominantly of high-angle brittle faults that termi-

nate downward at the turtleback detachments in both

listric and planar geometries (Fig. 4). This relation is

visible in the many small canyons that cut into the

footwalls of the Badwater and Mormon Point turtle-

backs and in the cliffs of Copper Canyon Formation

immediately north of the Copper Canyon turtleback.

Knott et al. (1997) argued that the multiple stran-

dlines at Mormon Point actually consisted only of

one strandline that had been offset by multiple nor-

mal faults. Hayman et al. (2003) found that, at Bad-

water and Mormon Point, these faults defined a

conjugate set with an approximately vertical maxi-

mum compressive stress. They estimated that these

faults accounted for approximately 600 m of Pleis-

tocene and younger horizontal extension at Mormon

Point. This interpretation contrasts with Keener et al.

(1993) who inferred that the Mormon Point turtle-

back fault was not active and was cut by a high-

angle fault at depth.

Each turtleback also displays some degree of

folding in its hanging wall. The most intensely

folded hanging wall exists at Copper Canyon,

where Drewes (1963) mapped a series of southeast-

plunging asymmetric folds over an area of approxi-

mately 10 km2. The most prominent anticline has a

mapped length of more than 3 km and verges north-

northeastward. At Mormon Point, Hayman et al.

(2003) reported a roll-over anticline adjacent to the

detachment fault. Immediately north of Badwater,

several kilometer-scale folds are visible in the Artist

Drive Formation.

3.3. Turtleback fault zones

The turtleback fault zones are the discrete brittle

surfaces that separate the ductilely deformed mid-

crustal rock of the footwalls from the upper crustal

rock of the hanging walls. Superb exposures of these

fault zones exist along the west front of each turtle-

back (Fig. 3A,B,C). In general, the turtleback faults

dip westward or northwestward at shallow angles

(b358), show evidence for a latest episode of predo-

minantly normal slip, and display a wide variety of

cataclastic fault rocks. Moreover, they each appear to

have been active as recently as the late Pleistocene

(Knott, 1999; Hayman et al., 2003). Outstanding

questions persist regarding their specific geometries

and their relations to the rest of the Black Mountains

frontal fault zone.


3.3.1. Fault geometries and relation to frontal fault

zone

The classic interpretation of the turtleback fault

geometries holds that they are antiformal and broadly

coincident with the antiforms in the footwalls (Curry,

1954; Wright et al., 1974; Otton, 1976; Holm and

Dokka, 1993). The turtlebacks do display geometries

suggestive of both antiforms and synforms, the

hinges of which are shown on Fig. 7A,B, but it is

not yet clear if these features actually represent

single, curved fault surfaces. Instead, several of the

larger bfoldsQ are probably the poorly exposed inter-

sections of two or more differently oriented, subpla-

nar fault surfaces. Miller (1991) argued this case at

Badwater (hinge a on Fig. 7A), as did Pavlis et al.

(1993) for Mormon Point (hinge f on Fig. 7B).

Additionally, the apparent folds at Copper Canyon

(hinges d and e on Fig. 7B) remain ambiguous, as

they could be interpreted as either intersecting faults

or kink-like folds in a single fault. Two minor folds

at Badwater (hinges b and c on Fig. 7A) do appear

to reflect true undulations in the fault, as the fault

trace is continuous across the fold hinges. The anti-

formal hinge (hinge c) coincides with the antiformal

hinge of the footwall.

The turtleback fault geometries are also important

to understanding the structural evolution of the fault

systems, as well as their genetic relation to the Black

Mountains frontal fault zone. At Badwater, the turtle-

back fault consists of at least three distinct fault

surfaces, which decrease in age but increase in dip

towards the west. Fig. 7A shows these surfaces as

faults #1, #2, #3. Miller (1991) attributed these rela-

tions to progressive rotation and abandonment of fault

surfaces through time, with fault #3, the range-bound-

ing fault, cutting fault #2. While the Miocene–Plio-

cene volcanic rocks of the hanging wall dip 20–308eastward to support this interpretation, Knott (1998)

and Hayman et al. (2003) noted that the Pleistocene

material of the hanging wall has not rotated. There-

fore, significant rotation of the turtleback fault and

footwall must have taken place prior to deposition of

the Pleistocene material, possibly before initiation of

slip on fault #2.

At Mormon Point, a gravity and magnetic study by

Keener et al. (1993) suggested a similar geometry to

Badwater in that the principal turtleback fault

appeared to be cut by a later, range-bounding fault.

Hayman et al. (2003), however, argued an alternative

geometry, where the exposed detachment fault (fault

#2 at Badwater) is the active, controlling structure,

and the present, high-angle, range-frontal fault termi-

nates downward at the detachment. To support their

interpretation, they noted that, at Mormon Point, the

hanging wall has been extended by approximately 600

m by minor normal faults that terminate downward

onto the detachment. We note, however, that 600 m of

slip is only about one third of the total likely slip

predicted across the Black Mountains frontal faults

during the time interval considered by Hayman et al.

(2003). Geodetic data and Quaternary offsets in north-

ern Death Valley (Williams et al., 1999) indicate the

Death Valley fault system should be extending the

region ~2.9 km/m.y.; i.e. since deposition of the

Bishop Ash (~600 ka) approximately 1.8 km of hor-

izontal motion should have occurred across the frontal

faults. Therefore, more slip has presumably accumu-

lated across buried faults, including the frontal faults,

than the exposed fault array at Mormon Point. Alter-

natively, the cumulative slip on hanging wall faults

reflects only a portion of the slip on the main detach-

ment fault (D. Cowan, personal communication,

2003).

We consider the relation between the turtleback

faults and the frontal fault zone of the Black Moun-

tains to be an open question. Hayman et al. (2003)

document features that require the turtleback faults to

be recently active. This activity makes it difficult to

envision the turtleback faults as cut by the frontal

fault zones, unless there is cycling back and forth

between gravitationally driven slip on the turtleback

faults and rooted slip on the frontal fault zones as

suggested by Miller (1999b) for Badwater. Miller

(1992a,b, 1999a,b) also showed that fault 2 of the

Badwater turtleback fault system coincides with the

Black Mountains range front about 2 km south of

Badwater, and cuts structurally upwards into volcanic

rocks immediately north of Natural Bridge Canyon

(Fig. 7A). In this way, the fault appears to be more of

an abandoned range front fault than a true detach-

ment. The Badwater turtleback detachment, if there is

one, is most likely fault #1 on Fig. 7A.

3.3.2. Kinematics and fault rocks

As inferred from slickenline orientations, the most

recent episode of slip on the turtleback faults has been


normal (Pavlis et al., 1993; Miller, 1999b; Hayman et

al., 2003). However, the faults likely had an earlier

history of right-lateral oblique motion, consistent with

the slip direction on the rest of the Black Mountains

fault zone (Struthers, 1990; Brogan et al., 1991; Slem-

mons and Brogan, 1999). The most recent, normal

slip therefore appears to be a departure from a more

long-standing norm.

At Badwater, evidence for earlier oblique motion

comes from the fault geometry, folded fault rocks,

slickenlines, and high-angle faults in the footwall

(Miller, 1999b). Hinge c in Figs. 3A and 7A, marks a

true fold in the fault plane that plunges about N 608W.

Miller (1999b) argued that for the hanging wall to

remain intact, slip had to parallel the hinge. Addition-

ally, a Hansen slip line analysis of mesoscopic folds in

the fault zone yielded a northwest-directed, oblique

vector, and slickenlines on crystalline rocks south of

hinge C indicated oblique motion. Miller (1999b) sug-

gested that because these slickenlines were on crystal-

line rock, they were more durable and therefore

reflected an earlier episode of slip than the dip-parallel

ones that are preserved solely on the underside of the

relatively soft hanging wall. Finally, the footwall is cut

by numerous, demonstrably oblique high-angle faults.

At Mormon Point and Copper Canyon, Pavlis et al.

(1993) used large-scale fault geometry and the distri-

bution of fault rocks to infer oblique motion. There,

calcite-rich implosion breccias exist within right-step-

ping planar sections of the turtleback faults. These

breccias form through episodes of sudden dilatancy

(Sibson, 1986), which would most likely occur

through seismic right-lateral oblique motions on the

fault zone. Additionally, Pavlis et al. (1993) and

Keener et al. (1993) documented oblique slip in sev-

eral localities along southwest dipping segments of

both the Mormon Point and Copper Canyon turtle-

back fault based on complex slickenline arrays in

fault-rocks below the fault surface. As at Badwater,

the footwalls in both Mormon Point and Copper

Canyon turtleback are cut by more steeply dipping,

oblique-slip faults that do not appear to cut into the

hanging walls (Pavlis et al., 1993).

Besides the implosion breccias near Mormon Point,

each turtleback fault displays spectacular exposures of

fault gouge, foliated and non-foliated cataclasite, and

microbreccia. These fault rocks have been the subject

of several studies, including those of fault behavior

(Miller, 1996), kinematics (Cladouhos, 1999a,b), and

fault rock evolution (Hayman, 2002). Fault rocks are

best developed below the discrete sliding surface of the

turtleback fault. They typically show a downward

structural progression from clay-rich gouge into

foliated cataclasite and then into a variable damage

zone of cataclasis and hydrothermal alteration that

extends up to several tens of meters into the footwall.

Cowan et al. (1997, 2003) showed that these zones

correlated with decreasing shear strains away from

the fault zone. Miller (1996) documented mesoscopi-

cally ductile flow in the fault gouge, and Cowan and

Miller (1999) showed that episodes of ductile flow in

gouge appear to alternate with episodes of discrete

sliding. Alternating localized and non-localized

events conflict with other studies of fault rock devel-

opment that argue for continuous localization of slip

on developing fault zones (e.g. Chester and Logan,

1986; Logan et al., 1992).

3.3.3. Estimates of slip

Because the brittle turtleback faults separate such

disparate rock types, slip estimates tend to be based on

indirect evidence. Several observations, however, sug-

gest that the amount of slip for each brittle fault is

limited to only a few kilometers at most. To a first

approximation, the most relevant feature is the Willow

Springs pluton, as visible portions of it exist in the

hanging walls and footwalls of the both the Badwater

and Copper Canyon turtlebacks (Figs. 1 and 7A).

At the Badwater turtleback, a small fault-bounded

exposure of mylonitic basement rock exists in the

hanging wall of Fault #2 near Badwater spring (Fig.

7A). Based on this exposure, Miller (1991) inferred

approximately 2 km of slip on Fault #2. As indirect

support of this estimate, we note that north of Natural

Bridge canyon, Fault #2 cuts upwards into the volca-

nic hanging wall, with rocks of the Artist Drive For-

mation in both its hanging wall and footwall (Fig.

7A). At Mormon Point, Knott et al. (2001a,b) esti-

mated 438 m of Pleistocene heave on the turtleback

fault based on an offset bed of Bishop Ash.

4. Discussion and conclusions

Among the structural features of Death Valley, the

turtlebacks contain some of the most critical informa-


tion relevant to testing models of crustal extension. In

particular, models are constrained by two types of

information from within the turtlebacks: (1) observa-

tions of the turtleback shear zones themselves, and (2)

implications of the basement-involved thrust faults of

the turtlebacks for the pre-extensional geometry of the

fold-thrust belt in Death Valley. This information has

implications that extend far beyond Death Valley, as

the region is frequently cited as a type example for

continental extensional tectonics.

Tectonic models for the Death Valley region

include both regional map-view reconstructions that

attempt to constrain the regional strains (e.g. Serpa

and Pavlis, 1996; Snow and Wernicke, 2000) and

models of the extensional process in cross-sectional

views (e.g. Holm et al., 1992; Miller, 1999a). Al-

though these approaches provide important illustra-

tions of the extensional process, they are limited to

two dimensions. In fact, the turtlebacks are complex

three-dimensional entities that formed over ~14 m.y.

of extensional overprinting on an older Mesozoic

framework. We submit that many of the controversies

regarding the Death Valley region in general, and the

turtleback systems in particular, stem largely from the

inability to communicate observations of these multi-

ple dimensions. Even the coauthors of this paper are

not in complete agreement on several details, but our

approach here is to clarify our view of the four-

dimensional history. Following previous presenta-

tions, we consider the problem in the context of

two-dimensional map and cross-sectional views, but

we acknowledge that these views are oversimplifica-

tions of the geometric evolution.

4.1. Map-view reconstructions of regional deformation

Several tectonic models for the Death Valley region

emphasize large-scale, map-view reconstructions

based on the correlation of regional structures and

stratigraphic assemblages within the Death Valley

region. These controversies were initiated by the cor-

relation of the Panamint thrust in the northern Panamint

Mountains with the Chicago Pass thrust system in the

Nopah and Resting Spring ranges (Wernicke et al.,

1988). Other workers, however, have challenged this

correlation on different lines of evidence (e.g. Wright et

al., 1991; Serpa and Pavlis, 1996; Miller and Prave,

2002). The resolution of this question is critical because

extensional magnitudes vary radically in different ver-

sions of these reconstructions.

As described byMiller (2003), the presence of base-

ment-involved thrust faults in the turtlebacks help

define the pre-extension configuration of the fold-

thrust belt in Death Valley, and therefore provide

keys to interpreting the crustal extension. As argued

below, their presence suggests that the Black Moun-

tains were a structural high prior to Tertiary extension.

It further suggests that restoration of the Cottonwood

Mountains above the Black Mountains is unlikely, yet

this configuration is required if the Panamint and Chi-

cago Pass thrusts were the same thrust displaced by a

master detachment system.

Evidence for a pre-extensional structural high in

the Black Mountains is supported by the general

absence of Paleozoic sedimentary rocks and the

deep stratigraphic level of the pre-Tertiary unconfor-

mity in the Black Mountains. In the Amargosa

Chaos, ~12 Ma sedimentary and volcanic rocks lie

depositionally on late Proterozoic to early Cambrian

sedimentary rocks (location bxQ on Fig. 1 inset;

Wright and Troxel, 1984; Topping, 1993). At the

Billie Mine in the northern Black Mountains, ~14

Ma sedimentary rocks of the early Furnace Creek

basin depositionally overlie early Cambrian rock

(Fig. 1; Cemen et al., 1985; Wright et al., 1999;

Miller and Prave, 2002). These contacts contrast

markedly with the northern Panamint Mountains

where late Paleozoic rocks lie directly beneath the

Tertiary unconformity. It is therefore likely that the

thrust systems now exposed in the turtlebacks pro-

duced significant structural relief above what is now

the Black Mountains and that the late Paleozoic

cover was largely stripped from the Black Mountains

prior to Tertiary extension. Correlation of the Pana-

mint thrust with the Chicago Pass thrust across this

structural high is suspect because structural relief

created by the thrust structures would disturb the

simple thrust belt geometries required by the correla-

tion. The correlation also places Late Paleozoic rocks

and thrust belt structures of the Cottonwood Moun-

tains structurally above and east of the turtleback

basement thrusts. This restoration conflicts with the

observed rocks below the Tertiary unconformity or

anywhere within the Black Mountains.

An alternative explanation, that the exposed Black

Mountains cover rocks are entirely allochthonous and


so can not reliably indicate a basement high, is unlikely.

In this scenario, the cover rocks should contain slices

that sample all parts of the hanging wall assemblage,

including late Paleozoic rocks (Fig. 10). Furthermore,

those rocks should be preserved beneath the Tertiary

unconformity. In fact, the youngest Paleozoic sedi-

mentary rocks recognized in the Black Mountains

are lower Paleozoic.

Moreover, the thrust structures among the ranges

are not age correlative. In the Cottonwood Mountains,

thrust structures are Permian to Triassic in age (Snow

et al., 1991), which is much older than the apparent

late Mesozoic–early Cenozoic age for the Black

Mountains thrust faults. Finally, the Paleozoic rock

of the Cottonwood Mountains is intruded by the

Jurassic Hunter Mountain batholith while the Proter-

ozoic and early Paleozoic rock of the Panamint Moun-

+

+

++

+

+

+

++

++

+

++

+ ++

++

++

++

+ ++

++

+

+

+

+

+++

+

+

+

+++

+++

++

+++

20 km

HMB

HMB

P

P

L

L

MC

MC

T

5 km

Panamint Mtns.

Panamint Mtns.

A

C

B

BMT

BMT

BM

Future site ofdetachment

A

A

A

Fig. 10. Schematic cross-sections from A to AV on Fig. 1 that account for p

depiction of present-day structure according to Rolling Hinge Model. B. D

Correlation of Chicago Pass and Panamint thrusts requires transport over b

Batholith on top basement rock of the Black Mountains.

tains are intruded by the Cretaceous Harrisburg Pluton

(Fig. 1). If either of these ranges originated above the

Black Mountains, then the plutons must have intruded

through them. However, no equivalent Mesozoic

intrusive rock have been recognized in the Black

Mountains (Fig. 10C).

4.2. Cross-sectional views of the turtlebacks

In cross-sectional view, the principal controversy

surrounding the turtlebacks centers on the theme of a

master detachment system with a rolling hinge (e.g.

Holm et al., 1992) vs. models that call on a polyge-

netic origin for the turtlebacks (e.g. Wright et al.,

1991; Mancktelow and Pavlis, 1994; Serpa and Pav-

lis, 1996; Pavlis, 1996). A related controversy con-

cerns the inferred connection, in space and time,

+ ++

++++

+

+ ++

+

Approx. basement-sed unconformity

Detachment fault

HMB P

LMC

CP

CP

CP

B

TB

Black Mtns.

Black Mtns.

Nopah R.

Nopah R.

T

A'

A'

A'

resence of basement thrust faults (BMT) in the Black Mountains. A:

epiction of present-day structure according to Pull-Apart Model. C:

asement high in Black Mountains and also places Hunter Mountain


between the turtleback systems and structurally

higher-level structures in the Black Mountains.

These structures include those in the Amargosa

Chaos area to the south (Wright and Troxel, 1984;

Topping, 1993; Topping in Holm et al., 1994a,b) and

low-angle normal fault systems that frame the Black

Mountains to the east. The structural relationships

between the Badwater and the other turtlebacks also

represent an important detail because the Badwater

turtleback is geochronologically distinct and lies in the

hanging wall of the Copper Canyon turtleback.

Existing Ar–Ar geochronology for the turtlebacks

can be interpreted to satisfy both models. Holm et

al. (1992) found that, from south to north, the

Mormon Point, Copper Canyon, and Badwater tur-

tlebacks cooled through 300 8C at about 8 Ma, 6

Ma and 13 Ma, respectively. They interpreted the

cooling ages at Mormon Point and Copper Canyon

as evidence of the hinge rolling northwestward from

Mormon Point to Copper Canyon and the much

older age at Badwater as an earlier, deep-seated

A

B

CB

++ + +

++ +++ +

++ ++ + +

++

+++

+++ +

+ +

+

Fig. 11. Schematic cross-sections to illustrate principal extensional events a

at mid-crustal shear zone. B: Intrusion of Black Mountains intrusive suite a

zone and recrystallized fabrics at Copper Canyon and Mormon Point tur

extensional faulting and ductile shear exhumed the intrusive suite and

approximate present-day exposure of metamorphic portion of each turtleb

event. We agree that the old age at Badwater

reflects an earlier event, but the 8 Ma and 6 Ma

ages at Mormon Point and Copper Canyon do not

require a rolling hinge. Instead, they probably reflect

diachronous cooling related to two simultaneously

operating processes: (1) cooling from the emplace-

ment of the ~11 Ma Black Mountains plutonic suite

and (2) unroofing of the turtlebacks by extension,

presumably at different times. In this context we

suggest that two features of the turtlebacks are

particularly relevant: the geometries of the turtleback

folds, and the location of much of the Badwater

turtleback beyond the northern terminus of the Wil-

low Springs pluton (Fig. 7A).

The latter feature suggests that intrusion of the

Willow Spring pluton dominated the cooling history

of the southern turtlebacks (Holm and Dokka, 1993;

Pavlis, 1996), but likely had little effect on the Bad-

water turtleback. As a result, the older cooling ages in

the Badwater turtleback reflect a minimum age for the

initial ductile deformation related to extension in the

Upper CrustalRock

C M

++

++

+ ++

++++ +

+ +

+

+

++

+ ++

++

++

+

+ + +++ +

+

Black Mountains Intrusive Suite

Mid-crustalshear zone.

t the turtlebacks. A: Prior to ~11 Ma, upper crustal faults terminated

t ~11 Ma. These sill-like plutons intruded along the top of the shear

tlebacks, but had little effect at Badwater Turtleback. C: Continued

underlying mid-crustal shear zone. Dark-shaded areas represent

ack footwall.


region. This early history was also present in the other

turtlebacks, but was largely obliterated at ~11 Ma

when the Black Mountains intrusive suite was

emplaced above them, completely recrystallizing

pre-existing microstructures and resetting all low-T

geochronometers. This conclusion is consistent with

onset of volcanism and incipient extensional basin

development prior to ~14 Ma (Wright et al., 1999).

For example, Miller and Prave (2002) found that

activity on the pre-13 Ma Badwater Turtleback fault

likely influenced sedimentation in the early Furnace

Creek Basin.

Similarly, the variability of fold geometries of the

turtlebacks, especially between the highly non-cylind-

rical Mormon Point turtleback and the nearly upright

Copper Canyon turtleback, are difficult to reconcile

with the concept of a single, master rolling hinge.

Moreover, their doubly plunging geometries are also

inconsistent, because a migrating hinge should leave

behind a single low-angle shear zone.

Our conceptual view of the turtleback evolution

is that they all share a common, early history as

parts of a mid-crustal shear zone at ~12–16 Ma that

at least in part reactivated a pre-existing basement

thrust fault (Fig. 11A). This shear zone locally

reached upwards to connect with supracrustal fault

zones as described by Miller (1992a,b, 1999a,b) at

the Badwater turtleback. When the Black Mountains

intrusive suite was emplaced at ~11 Ma, however, it

cut across many of these faults and recrystallized

fabrics in the subjacent ductile shear zone (Fig.

11B). Consequently, the thermally unmodified

older shear zone is only preserved at the Badwater

turtleback. At the Mormon Point and Copper Can-

yon turtlebacks, the older shear zone is now repre-

sented only by the thermally overprinted tectonite

fabrics of the southeastward plunging segments of

the turtleback antiforms. Continued crustal extension

caused the exhumation of the intrusive suite and

eventual formation of the brittle turtleback faults,

shown in Fig. 11C as dismembering the intrusive

suite. This view of the turtlebacks, in which much of

the ductile deformation occurred before emplacement

of the Miocene intrusives, and most brittle deforma-

tion occurred after their emplacement, differs signifi-

cantly from the rolling hinge model which requires that

most of the brittle and ductile deformation post-dated

intrusion.

Acknowledgements

Our views of the turtlebacks have benefited from

discussions with Darrel Cowan, Richard Friedman,

Nick Hayman, Dan Holm, Jeff Knott, Laura Serpa,

Bennie Troxel, Brian Wernicke, and Lauren Wright.

This manuscript was greatly improved through detailed

reviews by Darrel Cowan and Greg Davis.

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The Black Mountains turtlebacks: Rosetta stones of Death ... · The Black Mountains turtlebacks: Rosetta stones of Death Valley tectonics Marli B. Miller a,*, Terry L. Pavlis b a